The recent dramatic increase and geographic differences in frequency of reproductive diseases are likely influenced by changes in the environment, including perchlorate exposure. Perchlorate (ClO4-) is a persistent, chlorinated water-soluble contaminant that is pervasive in the United States. As a toxicant, perchlorate poses a major risk to human health through ingestion of contaminated water, food, and breast milk. Perchlorate is a known endocrine disruptor that competitively inhibits iodide uptake at the Sodium-Iodide Symporter (NIS) in the thyroid, thus hindering thyroid hormone synthesis. Studies demonstrate, however, that perchlorate exposure masculinizes both female and male stickleback fish (Gasterosteus aculeatus), leading to hermaphroditic females and males with testicular hypertrophy, results that are not predicted by a simple, direct thyroid- disruption mechanism. The goal of this project is to reconcile the dominant paradigm of perchlorate action - exclusively by disruption of NIS in the thyroid - with masculinization of behavior, physiology, and morphology in stickleback. The project's goal is to identify previously unsuspected pathways by which perchlorate may impact human reproductive health. Our working hypothesis is that perchlorate disrupts gonadal development by acting independently of the thyroid. Aim 1 will determine whether all observed phenotypic responses to perchlorate exposure in stickleback are mediated by the thyroid by rescuing thyroid hormone levels in perchlorate-exposed fish. Aim 2 will define the functional roles of NIS and NIS-paralogs in disruption of gonadal development by perchlorate using in situ hybridization to localize mRNA (Aim 2a), loss-of-function experiments to knock down expression of NIS and NIS-paralogs with morpholino anti-sense oligonucleotides and induced mutations using zinc finger nucleases (Aim 2b), and gain-of-function experiments by over- expressing the NIS and NIS-paralogs (Aim 2c). Aim 3 will determine the mechanism by which perchlorate alters sex differentiation using whole genome transcription profiling to determine which genes are early responders to perchlorate exposure, which are likely to be downstream genes, and whether responding genes are related to thyroid or gonad development. Quantitative PCR (qPCR) and in situ hybridization will verify expression profiling results. Significance. The proposed experiments will identify molecular and physiological pathways by which perchlorate disrupts gonadal development, whether solely via NIS in the thyroid or by other mechanisms. Because perchlorate is a pervasive contaminant in the U.S., our proposed work has direct implications for human health, particularly regarding thyroid diseases and disorders of sexual development.